docs: def _K(m):
      """ matrix K_m from Wiktorsson2001 """
      M = m*(m - 1)//2
      K = np.zeros((M, m**2), dtype=np.int64)
      row = 0
      for j in range(1, m):
          col = (j - 1)*m + j
          s = m - j
          K[row:(row+s), col:(col+s)] = np.eye(s)
          row += s
      return K